The present invention relates to a pressurized hydraulic piston-cylinder device with means for compensating internal volume variations and, more particularly, to a cap of a pressure cylinder of a pressurized hydraulic piston-cylinder device having a volumetric compensating device.
This invention has been developed within a research program of wider general scope relating to the manufacture of hydraulic self-regulating devices for regulating, by way of permanent adjustment, the length of the cable, or linking member, connecting a mechanism with the corresponding control member. An example of such self-regulating devices are the devices for regulating the length of the control cable linking the clutch mechanism with the clutch pedal. The purpose of this regulation of the control cable length is to compensate for the wear of the clutch plate lining during use.
A known hydraulic self-regulating device for the clutch control cable length is disclosed in Spanish patent P 9002014. It describes a device which comprises a main body member, or cylinder, housing therein a piston which comprises a check valve, a regulating rod which is connected at one end thereof to the piston and at the other end, which extends out from the main body member, it is connected to the control cable linking it with the clutch pedal and a liquid completely filling the interior space of the cylinder. Among its many possible applications, the device according to Spanish patent P 9002014 has proved to be highly efficient when used in the regulation of the length of the control cable extending from the clutch pedal to the pressure plate fork of the clutch mechanism. This regulation occurs by automatically varying the length of the portion of the regulating rod housed within the cylinder appropriately to compensate for the wear of the clutch. In other words, the length of the outwardly extending portion of the rod increases, which is equivalent to increasing the operative length of the control cable, whenever there is a physically noticeable wear so that the length of the cable increases, adjusting itself to the new situation arising from such wear.
The self-regulating device according to Spanish patent P 9002014 is provided with volumetric compensation means designed permanently and automatically to absorb the variations of volume affecting both the cylinder and the liquid contained therein owing to changes in the ambient temperature of the place where the device is installed. The compensation means are also designed to absorb the variations of volume caused inside the cylinder by the variations of the length of the rod portion contained in the cylinder.
Hereinafter, with a view to achieving adequate clarity of disclosure in the description, the useful internal volume of the cylinder is defined to be the same as the volume occupied by liquid plus the volume occupied by the volumetric compensation means housed in the cylinder, as measured under the original manufacturing conditions, i.e. with the piston, other mechanical members and a particular portion of the rod housed in the cylinder, at the temperature of manufacture.
In the hydraulic self-regulating devices such as the one described above, the useful internal volume of the cylinder does not remain constant, but quite otherwise. The useful internal volume varies, basically, depending on the variation of the rod portion contained in the cylinder and depending on the variation of the operating temperature of the cylinder. In the former case, an increase of the length of the rod portion contained in the cylinder, caused when the piston moves inwardly, implies a reduction of the useful internal volume while, on the contrary, a shortening of the length of the rod portion contained in the cylinder, caused when the piston moves outwardly, implies an increase in the useful internal volume. In the latter case, an increase of the cylinder operating temperature causes the cylinder and the liquid contained therein to expand, increasing the useful internal volume while, on the contrary, a drop in the operating temperature causes the cylinder and the liquid contained therein to contract, reducing the useful internal volume. In this respect, it should be noted that the coefficients of expansion of the cylinder and of the liquid are substantially different, with the coefficient of expansion of the liquid being notably higher than that of the cylinder so that an increase of the cylinder temperature will cause an increase of the liquid pressure against the inner surface of the cylinder and on the volumetric compensation means. On the contrary, a lowering of the cylinder temperature will cause a lowering of the liquid pressure against the inner surface of the cylinder and on the volumetric compensation means.
It will be gathered from the above that the volumetric compensation means included in the hydraulic self-regulating devices as described above absorb the variations of volume and, consequently, the variations in the pressure exerted by the liquid on the inside of the cylinder which affect both the cylinder and the liquid contained therein while the device is operating or even during the storage of the device prior to its assembly. In fact, at a constant temperature, a shortening of the length of the rod portion housed in the cylinder generates a depression causing a corresponding increase in the volume occupied by the volumetric compensation means, i.e., there is an increase in the useful internal volume while, on the contrary, an increase of the length of the rod portion causes a corresponding reduction of the volume occupied by the volumetric compensation means, i.e. there is a reduction of the useful internal volume. And, for a given position of the piston in the cylinder, i.e. for a given length of the rod portion housed in the cylinder, an increase of the cylinder temperature causes an expansion of the liquid greater than the expansion of the cylinder, which generates a pressure causing in turn a corresponding reduction of the volume occupied by the volumetric compensation means while, on the contrary, a lowering of the cylinder temperature causes a contraction of the liquid also greater than the contraction of the cylinder which, in turn, causes a corresponding increase in the volume occupied by the volumetric compensation means. Simultaneous changes of the length of the rod portion contained in the cylinder and of the cylinder temperature have a corresponding result, depression or pressure, which acts on the volumetric compensation means, increasing or reducing the compensation volume, as described above.